1. Field of the Invention
The present invention relates to a single mode optical fiber and, particularly, relates to a single mode optical fiber adapted for use in a high-speed digital transmission line.
2. Description of the Conventional Art
Conventional single mode optical fibers for communication (hereinafter referred as "SM optical fibers") are in most cases used at a wavelength of about 1.3 .mu.m or about 1.55 .mu.m, The use at the wavelength of about 1.55 .mu.m, however, has increased from the point of view of small loss. As for the refractive index distribution (hereinafter referred to as "profile") of the SM optical fiver adapted for this wavelength 1.55 .mu.m, there is an SM optical fiber comprised of an inner core having a high refractive index, an outer core having a lower refractive index than the refractive index of the inner core, and a clad having a lower refractive index than the refractive index of the outer core, as disclosed in Examined Japanese Patent Publication No. Hei-3-18161.
As described in the paper "1.55 .mu.m dispersion shift fiber", Fujikura Technical Report, pp. 1-7, No. 74, in such a conventional SM optical fiber, it has been considered suitably that the profile has the relations: EQU .DELTA.n.sub.1 =0.6%, EQU .DELTA.n.sub.2 /.DELTA.n.sub.1 &lt;0.17, and EQU 2a/2b=0.25 to 0.33
where the relative refractive index difference of the inner core with respect to the clad is represented .DELTA.n.sub.1, the relative refractive index difference of the outer core with respect to the clad is represented by .DELTA.n.sub.2, the diameter of the inner core is 2a, and the diameter of the outer core is represented by 2b.
Further, the specific refractive index of the inner core .DELTA.n.sub.1 and the specific refractive index of the outer core .DELTA.n.sub.2 are expressed as follows: EQU .DELTA.n.sub.1 =(n.sub.1.sup.2 -n.sub.0.sup.2) and .DELTA.n.sub.2= (n.sub.2.sup.2 -n.sub.0.sup.2)/2n.sub.2.sup.2
In Unexamined Japanese Patent Publication No. Sho-62-291605, it has been made suitable to have the relations: EQU .DELTA.n.sub.2 /.DELTA.n.sub.1 0.1.about.0.4, and EQU 2a/2b=0.3.about.0.6.
In the conventional art described in these literatures, in view of the reduction of the bending loss of light, the refractive index of the inner core is constant or decreases uniformly from the center of the inner core toward the outer core. However, since the optical fiber is used for short distance transmission or relay transmission, it is possible to neglect the dispersion of light pulses due to a nonlinear effect. Accordingly, the requests to enlarge mode field diameter (MFD) and to reduce the dispersion slope, as will be described later, are not so severe.
If the density of light power in the optical fiber increases in the case where relayless long-distance transmission is to be performed by using optical amplifiers, the spread (dispersion) of light pulses due to a nonlinear effect cannot be neglected. It is therefore necessary to reduce the light power density, but because the reduction of the total quantity of light power for this purpose brings an disadvantage such as increase of bit error or the like, it is effective to widen the light power distribution in the sectional direction of the optical fiber.
The diameter of the light power distribution, that is, the diameter in which light power becomes 1/e as much as the center (maximum) of light power, is called MFD (Mode Field Diameter). Accordingly, widening the light power distribution in the sectional direction of the optical fiber causes another problem to increase MFD.
In addition, digital transmission is also being proceeded at a high speed in accordance with the introduction of light amplifiers. In such a high-speed digital transmission line, the distance between light pulses is so small that there also arises a problem in which the width of the light pulses is expanded by the wavelength fluctuation of light emitted from a light source if the dispersion slope of an SM fiber used there is large.
Generally, light having a plurality of wavelengths is ordinarily emitted from the light source. Indeed dispersion does not occur since a plurality of light beams of the respective wavelengths are propagated at almost an equal speed. However, the propagation speed in the SM fiber depends on wavelength, so that dispersion occurs when the central wavelength gets out of zero-dispersion wavelength due to the wavelength fluctuation of the emitted light. The size of this dispersion takes a negative value when the wavelength is shorter than the zero-dispersion wavelength, and takes a positive value when the wavelength is longer than the zero-dispersion wavelength. The size of the dispersion relative to the wavelength changes in an approximately straight line near the zero-dispersion wavelength. When the dispersion slop, which means the inclination of wavelength dispersion in the zero-dispersion wavelength, is small, the size of the dispersion is small even in the position which is apart from the zero-dispersion wavelength.
In a conventional 1.55 .mu.m SM optical fiber profile, at least one of dispersion slope and bending loss is increased when MFD is increased, so that the expansion of the MFD and the reduction of the dispersion slope cannot be compatible with each other. Therefore, there has been a problem on practical use.
For example, while 20 mm.phi. bending loss is not more than 1 dB when MFD is in the normal region of 7.5 to 8.0 .mu.m.phi., it takes 20 dB or more when 9.0 .mu.m.phi.. The MFD had better be increased in order to restrain the dispersion caused by nonlinear effects. However, if the 20 mm.phi. bending loss is above 20 dB consequently, the possibility to increase the loss in making the fiber into a cable is so high as to cause a problem on practical use.
In a conventional profile, assume that zero-dispersion wavelength is fixed into a predetermined wavelength near 1.55 .mu.m, bending loss satisfies the relation 20 mm.phi. bending loss .ltoreq.1 dB/m as well as conventional one, and cutoff wavelength is set to not more than operating wavelength. In order to increase MFD, it is necessary to reduce relative refractive index difference .DELTA.n.sub.1 of an inner core, and increase diameter 2b of an outer core. However, dispersion slope increases then. For example, if the MFD is increased from 7.5 .mu.m.phi. to 8.0 .mu.m.phi. when the zero-dispersion wavelength is 1.56 .mu.m, the dispersion slope is changed from 0.06 ps/nm.sup.2 /km or more to 0.075 ps/nm.sup.2 /km or more, so that the lower limit is increased. Therefore, there is a disadvantage with respect to the wavelength fluctuation of a light source.
Further, it is necessary that the zero-dispersion wavelength is set to be longer than the used wavelength in order to prevent the mixture of four light waves in the case where an optical amplifier is used, and it is therefore necessary that the ratio .DELTA.n.sub.2 /.DELTA.n.sub.1, that is, the ratio of the average value .DELTA.n.sub.2 of the relative refractive index difference of the outer core to the average value .DELTA.n.sub.1 of the relative refractive index difference of the inner core is set to be smaller, so that increase of bending loss arises from this point of view to thereby bring a disadvantage in the conventional profile.